A 3 line header is returned first giving the start frequency f
1
, the finish frequency f
2
and
the number of samples n with each value in ASCII decimal being terminated with a CR
character (in current software n is always 256, but this should not be assumed). This is
followed by n samples of binary data, with each sample taking 2 bytes. Note that this
binary data may contain any 8 bit number (0-255) so it is important that the operating
system or communications library treats it as raw data without processing end-of-line, or
end-of-file characters.
For each sample pair, the first byte is the integer part (in dB), while the next byte is the
fractional part (1/256dB) using two’s complement. For example, the bytes $FF,$C0
would represent -0.25, while $05,$80 represents +5.5dB (‘$’ indicates hexadecimal). The
levels are all absolute levels in dBu (not normalised) and the remote program should
subtract the appropriate value to normalise the results as required (sample number 145 for
1kHz or 112 for 400Hz normalisation with sweep segments P,Q,R,S,U and X).
The frequency at which the ith sample was measured, can be assumed to be:
where i is in the range 0 to n-1. ie the frequency is exponential with time.
Note for advanced users: The LA101 does not generate truly exponential sweeps, as it
in fact generates each frequency for a constant time, and the 32 frequencies per octave
which the LA101 uses are not spaced exponentially (although the error is small). The
LA102 corrects for this by using and displaying the actual frequency generated by the 101
at the time of that sample. Refer to SW command for further details.
Example
To read all of the results from an LA102 a program should perform the following steps:
1. Put the LA102 into remote mode.
2. Send an ID? command to read the identity of the LA102.
3. Read the identity into a string (terminated by a CR character).
4. Ensure that the LA102 has V6.0 or later software. Do this by scanning the identity
string until a capital ‘V’ is found, and read the following digits (one or two digits will
precede the decimal point). If this number is less than 6, the LA102 does not support this
format and the results cannot be read, so generate an error and go to step 9.
5. Send an SR?n command (n is usually 51 or 59, but could be 1 or 9 when reading
simple results).
6. Read characters into text buffer up to and including the terminating ASCII 26
character.
7. Scan the text buffer for ^ characters. For each one that is found, read the following
graph handle (note that the graph handle may be over 9, so all subsequent digits should be
147
9. Remote Control
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